1. Field of the invention
This invention relates to a slider for use in mechanical seals, bearings, valve seats, etc. under severe atmospheric conditions, and more particularly to a high performance slider of silicon carbide sintered compact of which the friction coefficient is reduced in addition to improving the lubrication property, mechanical strength, abrasion resistance, etc.
2. Prior art
A high-density silicon carbide sintered compact (hereinafter referred to as "SiC sintered compact") has long been known for its excellent property such as excellent resistance to heat, abrasion and corrosion, etc., and is one of the useful ceramics used, for example, in gas turbines. In particularly the SiC sintered compact has already been put in practical use as sliders for mechanical sealing and is now considered one of the high performance slider materials with the aforementioned excellent property.
With an increasing of demand for highly effective seals, bearings, etc. under the background of remarkable technological development, a higher sealing performance is now required in mechanical seals, a higher lubricating performance in bearings and a higher sliding performance in sliders composed of the SiC sintered compact.
Looking at known SiC sintered compacts actually used, they are obtained, for example, by a step of admixing a fixed amount of boron compound and/or aluminum compound, each serving as a sintering accelerator, with a fixed amount of carbon serving as a sintering aid, a step of molding into a compact, and a step of sintering the compact at a high temperature of not lower than 2000.degree. C. under an inert atmosphere. However, free carbon is found not only on the surface of SiC sintered compact but also at grain boundaries thereof due to a sublimate decomposition reaction of SiC itself at the sintering step. As a result, there exists when viewed microscopically crystal grains of SiC and grain boundaries of carbon on the surface of the high-density SiC sintered compact used for mechanical sealing. In other words, there is a segregation of carbon at fine cavities on the surface of the compact.
As is well known, carbon has a lipophilic property as well as relatively high hydrophilic property, and therefore it is usually possible for a high-density SiC sintered compact having the aforementioned microscopic carbon on the surface (i.e. sliding surface) to exhibit a high performance as a slider for mechanical sealing of fluid under a fluid lubrication. Due to a high temperature or a high viscosity produced in the liquid to be sealed, however, it is sometimes very difficult to stabilize a fluid lubrication film formed on the sliding surface in the case of a high-density SiC sintered compact of the aforementioned composition, and as a result, there is the possibility of abnormal abrasion or an increase of the friction coefficient on the sliding surface occuring which would cause thereby a decline in the sealing performance.
Accordingly, in the case of a slider composed of such a high-density SiC sintered compact, it is necessary to increase carbon content in order to improve stabilization of the fluid lubrication film on the sliding surface. But when free carbon (fc) is excessively increased in the SiC sintered compact, there exists a problem in that a desired dense packing (i.e., high density) of the sintered compact is not gained due to the increase of the carbon crystal structures and as a result the combination strength of the grain boundaries is considerably lowered, eventually declining the abrasion resistance to be exhibited by the slider.